Acrylic adhesives (the term “pressure sensitive adhesives” refer to “pressure-sensitive adhesives”) have an excellent heat resistance and weather resistance derived from the structure thereof and therefore have been widely used in adhesive materials, such as adhesive tapes and adhesive sheets, as compared with rubber adhesives in recent years. In general, acrylic adhesives are mainly classified into solution-type adhesives, emulsion-type adhesives, and hot-melt adhesives on the basis of the forms thereof. Among these adhesives, solution-type adhesives each contain an organic solvent used for dissolving materials. Solution-type adhesives are applied to substrates, and the organic solvent is evaporated to form adhesive layers on the substrates; hence, the impacts of the organic solvent on environment have been problematic. Emulsion-type adhesives are applied to substrates in the form of emulsion, and water is evaporated to form adhesive layers on the substrates, which generates problems of a wastewater treatment and a large energy consumption for drying. Hot-melt-type adhesives are applied to substrates in a thermally melted state to form adhesive layers on the substrates and are therefore less harmful to environment as well as excellent in safety. In addition, since the hot-melt-type adhesives eliminate use of organic solvents and a step for drying water, which are necessary in solution-type adhesives and emulsion-type adhesives, they enable high productivity of adhesive materials and have advantages in terms of energy conservation and resource saving.
Adhesives have been demanded to have further advanced performance these days, and development of a hot-melt-type acrylic adhesive having both adhesive properties, such as a cohesive force and an adhesive force, and hot-melt coatability has been highly demanded. Patent Literatures 1 and 2, for example, each disclose a hot-melt adhesive using an acrylic block copolymer; however, a adhesive having a small viscosity has not been known. Furthermore, in the case of using a adhesive containing, as a polymer component, only an acrylic block copolymer that is in the form of solid at room temperature, the hot-melt coatability is not necessarily sufficient, which is problematic.